Bugs in the implementation of the triangular-shaped interferometer
Hi,
I believe I have found two bugs in the implementation of the triangular-shaped interferometer.
The first issue concerns this test, where the angles passed to this function should be given in radians, as also stated in [1]. However, in the current implementation, the last two latitude values are provided in degrees.
When the angles are passed correctly, the test fails. For example, if the position of the first vertex is set to (40.516°, 9.416°) with x_arm_azimuth = 70.5674°, the distances between the vertices are:
-
ET-1
↔️ ET-2: 9.06 km -
ET-1
↔️ ET-3: 9.76 km -
ET-2
↔️ ET-3: 7.70 km
I believe this test failure is due to an incorrect implementation of the functions updating latitude and longitude, which differ from those provided in [1]:
const φ2 = Math.asin( Math.sin(φ1)*Math.cos(d/R) +
Math.cos(φ1)*Math.sin(d/R)*Math.cos(brng) );
const λ2 = λ1 + Math.atan2(Math.sin(brng)*Math.sin(d/R)*Math.cos(φ1),
Math.cos(d/R)-Math.sin(φ1)*Math.sin(φ2));The only difference to keep in mind is the definition of the bearing angle: in bilby it is defined counterclockwise from east, whereas in [1] it is clockwise from north. This leads to the following conversion:
brng = 90 - x_arm_azimuthUsing the above expressions, the coordinates are recovered correctly. I also include a map illustrating this result: the blue dots represent the incorrect coordinates, while the red dots show the corrected ones.
I believe this also resolves this issue, since, when using the Virgo site coordinates as the starting point, it becomes possible to recover the positions of the other two vertices correctly, as defined in LAL.
I am still verifying this, but I don’t believe the bug has any practical impact on parameter estimation results.
I’d be happy to contribute and open a pull request if useful.
I am tagging a few people who might be interested in the matter @jacopo.tissino @sylvia.biscoveanu
Thanks a lot in advance.
Kind regards, Filippo